The principal part of the organic content of oil shales is an insoluble solid polymeric material normally referred to as "kerogen". A small amount of a soluble organic material, bitumen, is usually found associated with the insoluble kerogen. To generate usable organic products from the shale, the most common process is the thermal decomposition of the initial organic fraction of the rock. At the temperatures conventionally employed, e.g., 500.degree.-1000.degree. C., organic material vaporizes from the shale, leaving a solid organic residue on the inorganic fraction of the shale, the condensible portion of the volatile material in shale oil.
Kerogen in oil shale decomposes by means of the following two-step reaction:
Kerogen.fwdarw.Bitumen.fwdarw.oil+gas+spent shale. Under conventional processing condition, the shale is heated non-isothermally at a rate of a few degrees per minute up to temperatures ranging from 500.degree.-1000.degree. C. It is known, however, that almost all of the conversion occurs between 350.degree. and 550.degree. C. Additionally, the higher the heating rate, the higher the oil yield. Above 550.degree. C., the oil begins to degrade (and the mineral carbonates decompose, consuming energy). The principal nonoxidative ways by which the liberated shale oil can be degraded are cracking and coking reactions. Cracking is defined herein as vapor phase bond fission reactions that eventually lead to a distribution of molecular units (mostly units smaller than the original molecule), plus some carbonaceous residue. Coking is defined herein as liquid or condensed phase reactions resulting in the fusion of two or more molecular species with the ultimate formation of a carbonaceous product, plus minor amounts of lower molecular weight gases.
There are generally two conventional methods for retorting oil shale: combustion front heating; and solid/solid heat transfer. Combustion front heating provides a slow heating system, resulting in the production of less oil, due to some oil degradation. With solid/solid heat transfer, a solid loop is utilized to effect a heat transfer to the oil shale. The process is more efficient, but very expensive.
Focused solar radiation can be employed in the oil shale retorting process to effect rapid heating and hence a high yield of oil. Additionally, in recent years it has also become recognized that solar energy may be used as the heat source for driving the endothermic char-gasification reaction. For example, U.S. Pat. Nos. 3,993,458 (M. H. Antal, Jr.) and 4,229,184 (David W. Gregg) both disclose the use of solar energy in coal gasification.
Although the use of solar radiation in oil shale retorting is attractive in the sense that the shale can be rapidly heated to a retorting temperature, resulting in a large yield of oil produced, exposure of the oil shale to direct solar radiation during the entire retorting process presents limitations. After the retorting temperature has been reached, the oil shale begins to re-radiate the solar radiation back out through the window from which the radiation originally entered. Additionally, hot spots may form on the oil shale, causing oil degradation and mineral carbonate decomposition.